Stabilizing Metallic Zn Electrode Using Organic Acid Additives in Aqueous Zinc-Ion Batteries

Abstract

Aqueous zinc (Zn)-ion batteries (AZIBs) have been paid attention to as safe, economical, and high-energy-density storages through multiple electron transfer. Although conventional alkaline solution-based AZIBs are primary batteries, recent studies demonstrated the reversible Zn2+ deposition and stripping processes enabled in mildly acidic solutions. It promises the development of rechargeable AZIBs and viable applications for energy storage systems (ESSs). However, metallic Zn employed as the negative electrode has suffered from severe corrosion and precipitation of electrolyte salts in this low pH condition. The Zn electrode undergoes a hydrogen evolution reaction (HER) as Zn2+ is dissolved from the Zn surface. The corrosion leads to a pH rise, precipitating electrolyte salt as zinc hydroxide forms. For example, hexagonal plates of zinc hydroxide sulfate hydrate (Zn4SO4(OH)6∙xH2O, indicated as ZHS) grew on the Zn electrode in 1 M ZnSO4 solution when pH approached ~5.4. The insulating and randomly oriented ZHS increased the surface resistance and caused the non-uniform Zn deposition.These challenges are mitigated by adding organic acid to the 1 M ZnSO4 solution. As the HER raised pH, the acid was promptly deprotonated, then coordinated with Zn2+. This process caused the thin film formation consisting of three-dimensional zinc glutarate. The zinc glutarate protected the Zn electrode and impeded the electrode corrosion and the ZHS precipitation. We demonstrated a uniform Zn plating and stripping process with 10 mM of organic acid in contrast with dendritic and dead Zn growth in the absence of the additive. Galvanostatic tests of symmetric Zn cells revealed over 1000 h cycles with the additives at a current density of 1 mA cm-2 and a limited capacity of 1 mAh cm-2. In contrast, the Zn cells with the additive-free electrolyte solution exhibited 15 times lower cyclability as micron-scale ZHS plates covered the electrode surface. Our strategy using the cheap additives is feasible to use grid-scale ESSs and shows significantly improved cycling performances. I will present details of Zn surface reactions using organic acid additive and corresponding electrochemical performances in this presentation.